1. Field of the Invention
The present invention relates generaly to a novel system for radioactive well logging and more particularly to systems for control of the operation of a neutron generator to effect well logging data acquisition enhancement.
2. The Prior Art
In one illustrative process for radioactive well logging described in "Laboratory Studies of a Pulsed Neutron-Source Technique in Well Logging," Technical Note 2061, Journal of Petroleum Technology, 1960, by J. Tittman and W. Nelligan, the presence of hydrocarbons can be determined by the detection of gamma rays produced by inelastic scattering of fast neutrons from carbon nuclei in the earth formation under investigation. The neutrons are generated by fast neutron sources of the accelerator type, such as the 14 MeV generators disclosed in U.S. Pat. Nos. 3,461,291 to C. Goodman and 3,546,512 to A. H. Frentrop. These neutron generators are operated so as to produce fixed width neutron bursts generally of the order of 18 microseconds at a fixed repetition rate of the order of 10 kHz. A detector is gated coincidentally with operation of the fast neutron source to obtain an inelastic gamma ray spectrum.
The detector, which may be a crystal scintillator coupled to a photomultiplier, receives the radiation occasioned in the formation by the fast neutrons and generates a corresponding pulse. The output of the detector is coupled into an amplifier whose output in turn is applied to a discriminator employed as a threshold device to remove pulses having amplitudes below and above given threshold values which might otherwise cause pile-up on transmission lines. The output of the discriminator is applied to further processing apparatus for deriving therefrom information regarding, for example, the presence of hydrocarbons in the formations.
It has also been proposed, in U.S. Pat. No. 3,885,154 for R. B. Culver, to irradiate a formation surrounding a borehole with discrete bursts of fast neutrons at a fixed repetition rate; the duration of each burst being 10 microseconds. This burst duration interval is chosen, along with a corresponding detector gate interval, to reduce inelastic measurement contamination by capture gamma ray background. Additionally, Culver proposes to further reduce the capture gamma ray background, without reducing the inelastic gamma ray production, by shortening the burst duration interval as a function of the detected radiation.
While the scheme for neutron generator operation proposed by Culver effects a reduction of capture gamma ray background in the detected inelastic measurements, it will be appreciated that no enhancement in the data acquisition rate is effected over the fixed burst duration interval operation scheme since the burst repetition frequency is constant and the resulting mean burst width is narrower than the fixed burst width. Moreover, since the ratio of the generator "on" interval to the generator "off" interval "duty cycle" for the operation of the neutron generator is variable, the generator is often not operated at the "optimum" duty cycle for efficient operation of the generator.